This disclosure relates generally to an arrangement for improving an accuracy of a pressure measurement made of a breathing gas including drops of water or humidity flowing along a flow channel. Also this disclosure relates to a flow sensor for a flow rate measurement of a breathing gas including drops of water or humidity.
In hospitals, during intensive care and operations, a respiratory apparatus must mostly be used to take care of a patient's respiration. An unhindered flow of gases into and out of the patient's lungs is naturally of vital importance. A condition of gas channels can be monitored both by measuring concentrations of inhaled and exhaled gases and by measuring a flow and pressure of the gases. Especially, monitoring of the carbon dioxide content of an exhalation gas is widely used as a routine in operating theaters and now more frequently also in the intensive care unit. However, the flow and pressure measurements are an essential additional function both in respect of safety and because they make it possible to calculate quantities descriptive of the mechanical operation and respiratory metabolism of the lungs.
In principle, there are many applicable types of flow sensors. However, measurements under clinical conditions involve many problems. The flow is measured from the end of a so-called intubation tube inserted into the patient's windpipe. The sensor is therefore exposed to humidity, condensed water, and mucous secretions coming from the windpipe. It is clear that such soiling is likely to affect the operation of most types of flow sensors. The main types of flow detector and their principles are presented e.g. in the publication Doebelin: Measurement Systems, McGraw-Hill Kogakusha, 1976. Flow sensors based on differential pressure are best suited for clinical use. The flow in the respiratory tube may be laminar or turbulent. In the case of laminar flow, the pressure difference across a flow restricting element placed in the tube is directly proportional to the flow. In the case of a turbulent flow, the pressure difference depends on the square of the flow. In practice, the relation between the measured pressure difference and the flow always has to be empirically determined for a specific mechanical sensor construction. The flow sensors currently used are generally made of plastic and can either be disposable or reusable. Water condenses as droplets on the interior walls of the sensor and this reduces the cross-sectional area of the flow sensor with subsequent increase in the measured pressure difference and then also the flow signal. This problem with a drifting signal can be severe especially when using the flow sensor under very humid conditions typically for several hours.
One way to eliminate the drift is to heat the sensor to a temperature sufficient to prevent condensation. However, this method requires a heating element and electrical connectors. Another way of dealing with the problem is to treat the inner surface of the sensor in order to reduce the contact angle of the droplets so that they flow out and wet the surface of the sensor evenly. According to another similar method the inner surface is treated with a material retaining water inside it. Any of those methods takes care of the drift in the flow signal. However, there is also a problem with the pressure measuring tubes connecting the differential pressure orifices in the flow path with the differential pressure sensor. Small amounts of water can enter these narrow tubes even if there is no net flow in the tubes, only a minor to-and-fro fluctuation as a consequence of pressure changes in the respiratory tube. If the water clogs either one of those tubes there will be a fast change in the pressure difference to a new level, which is incorrect. Most frequently, the water enters the orifice closest to the patient because of the more humid exhaled gas. Condensed water droplets also often detach from the walls in the respiratory tube when the patient is turned or moved and may as a consequence flow into the orifices.
One remedy is to heat the orifices and tube entrances, but this method is not customer friendly as mentioned above. Another method is to design the sensor adapter in such a way that water is unlikely to enter the orifices. This may work in many cases, but under very wet conditions like prolonged use in critical care, water may still enter the pressure measuring tubes and cause erroneous signal levels. One method would be to cover the orifices with a hydrophobic porous membrane. Even if no water then can enter the pressure measuring tubes the membrane will cause a pressure drop which may not be constant under very wet conditions or when the membrane gets soiled with mucus. This would induce an error in the measured flow value. Still another method uses a minimal purge flow in the measuring tubes away from the pressure sensors. This method mostly works but it is technically a more complicated and also expensive construction. Further, it could contaminate the respiratory tube.